Integrated circuits in GaAs fabricated for optical and/or electronic device applications frequently incorporate features, e.g., mirrors/facets, channels and mesas, produced by dry etch processing. (The term GaAs as used herein includes all compounds, crystalline and polycrystalline, doped or undoped, containing gallium and arsenic with or without additional elements.) Often, in integrated circuits of this type, overlayers (metals and/or dielectrics) in device elements are required between dry etch features. This is true for certain active device elements such as laser diodes and photodiodes where metal overlayers are required as top surface electrical contacts. In a typical fabrication scheme for devices of this type electrical contacts are applied and patterned by lithographic means through separate process steps near the end of the process sequence. A method which permits the patterning of deposited overlayers, e.g., top surface contacts, concurrent with the patterning of other features during a process sequence, e.g., etch features, provides an advantage by reducing the number of process steps required Implementation of such a method, however, requires use of a suitable mask, i.e., a highly durable and process integrable mask, which will protect the overlayer from damage during patterning and subsequent processing. With reference again to top surface electrical contacts as deposited overlayers on integrated laser and photo diode elements, subsequent processing steps can include: an anneal at an elevated temperature to produce ohmic contact between the overlayer and the GaAs, lapping and polishing of the substrate backside, application of a backside electrical contact, another anneal at an elevated temperature to produce ohmic contact between the backside contact and the GaAs, one or more etch processes to produce isolation features/facets in the GaAs, removal of the protective mask, plus a number of cleaning procedures in solutions containing acids, bases, or solvents.
To prepare etch features in GaAs with small, often sub-micron size dimensions, processes which provide a high degree of anisotropy are often required. Wet processes are frequently unsuitable as they etch isotropically or crystallographically and undercut the mask. Dry processes, on the other hand, can, under suitable conditions, etch anisotropically and prevent mask undercut.
The majority of the dry processes that provide anisotropy in GaAs are ion-based techniques which utilize chemistry to provide some form of reactive assistance. The most common of the dry etch processes include: Reactive Ion Etching (RIE), Reactive Ion Beam Etching (RIBE), and Ion Beam Assisted Etching (IBAE) [also known as Chemically-Assisted Ion Beam Etching (CAIBE)]. The chemistry utilized by these techniques for reactive assistance enhances etch rates, forms volatile etch products, and minimizes (relative to non-chemically assisted processes) damage to the GaAs surface by energetic ions, neutrals, and/or radicals. In the dry etch processing of GaAs, ambients containing chlorine (atoms, molecules, neutrals, radicals) have been found quite useful for providing the necessary reactive assistance.
Some form of mask is required to protect desired regions of a GaAs surface (which may or may not contain a deposited overlayer) when the surface is subjected to an etch process, either wet or dry. Masks which are "durable" and "process integrable" are of particular value.
The term "durable" as used herein defines the resistance of the mask to erosion during an etch process. An ideal mask is durable to the extent it will not erode or change form during an etch process. For a mask to exhibit significant durability in the etch processing of GaAs, the ratio of the etch rate of the GaAs to that of the mask, i.e., the selectivity of the etch, must be high. Masks of materials of low durability are unsuitable for several reasons. First, the edge quality of etch features decreases as mask thickness increases. Second, mask features with lateral dimensions smaller than the thickness of the mask are unstable and can break away or shift position during processing. Finally, during dry etch processing, mask erosion, especially of edges, can redeposit mask material into unwanted regions and degrade overall etch quality and uniformity.
For a mask to be "process integrable" the mask must be capable of withstanding a variety of process steps in the processing of GaAs beyond those directly associated with dry etch processing. These steps would include; thermal processes which challenge the adhesion and stability of the mask and various cleaning processes in solutions containing acids, bases or solvents which challenge the reactivity of the mask. A process integrable mask, in summary, is a mask suitable for insertion into a process sequence without introducing contamination or becoming ineffective. It also must be easy to remove.
In the dry etch processing of GaAs using chlorine for chemical assistance, few materials are known which when applied as thin layers (ca. 0.1 um) offer the durability to survive an etch of from a few to many microns. Metals such as nickel (with titanium underlayer) and chromium and salts such as aluminum fluoride and strontium fluoride have been used with varying degrees of success. Wet techniques are frequently required following dry etch processing to effect complete removal of these materials. A definite need exists for dry etch masks of high durability which are convenient to apply, pattern, and remove.